Get Current User of Drupal 9 Externally

You have a stand-alone application that is not a Drupal module but resides in a Drupal sub-folder. And you want Drupal to manage your users. You want to access the currently logged-in Drupal user from your application. The following function will give you the current  user id, name, email and roles:

use Drupal\Core\DrupalKernel;
use Symfony\Component\HttpFoundation\Request;

/**
 * Get Drupal current session user details.
 * Passing Drupal folder, or its relative folder such as '..'
 * when it is called from a Drupal sub-folder.
 * Return ['id', 'name', 'email', 'roles']
 */
function get_drupal_current_user($drupal_dir) {
	// Change the directory to the Drupal root.
	chdir($drupal_dir);
	$drupal_root = getcwd();

	if ($drupal_root === false)
		return [];

	$autoloader = require_once 'autoload.php';

	$kernel = new DrupalKernel('prod', $autoloader);

	$request = Request::createFromGlobals();

	// Emulate Drupal /index.php to get the current user id
	$request->server->set("SCRIPT_FILENAME", $drupal_root . "/index.php");
	$request->server->set("REQUEST_URI", "/");
	$request->server->set("SCRIPT_NAME", "/index.php");
	$request->server->set("PHP_SELF", "/index.php");

	$response = $kernel->handle($request);
	
	$user_id = \Drupal::currentUser()->id();
	$acct = \Drupal\user\Entity\User::load($user_id);
	if ($acct != null) {
		$user_name = $acct->getDisplayName();
		$user_email = $acct->getEmail();
		$user_roles = $acct->getRoles();
	}
	else {
		$user_name = "";
		$user_email = "";
		$user_roles = [];
	}
	
	$kernel->terminate($request, $response);
	
	return ['id' => $user_id, 'name' => $user_name, 'email' => $user_email, 'roles' => $user_roles];
}

It emulates Drupal 9 index.php. It has to emulates index.php otherwise Drupal will create an anonymous session. Technically, you are already inside Drupal and you can call any Drupal function from within the code above.